The present invention relates to an ATM (Asynchronous Transfer Mode) transmission apparatus and its exchange, and in particular to a cell routing method and apparatus. The ATM described in the present application conforms to CCITT Recommendation I. 361.
In a duplex system established at the present time as a fault tolerance technique, routine table rewriting is performed by preparing two routing tables, i.e., an active routing table and a stand-by routing table and by switching the active table over to the stand-by table in response to a signal fed from a control unit.
In routing table rewriting in an ATM switch, a VPI (Virtual Path Identifier) conversion table or a VCI (Virtual Channel Identifier) conversion table must also be rewritten. However, it is practically impossible to simultaneously execute information rewriting upon these two tables. Therefore, there is a possibility that cell routing is performed transitionally while tables before and after the rewriting are mixedly present. That is to say, individual cells are stored in cell buffers dispersedly disposed in various locations and those cells are outputted to succeeding stages after time delays. Therefore, it is impossible to execute the table selection switching throughout the apparatus by means of external control as in the prior art. Because time delays are typically statistical, and hence cell routing may be conducted in a state of noncoincidence between contents of a referred table and routing requests of individual cells. Since such a phenomenon results in transmission of unnecessary cells onto unexpected highways, it is desired to prevent such a phenomenon from the viewpoint of transmission quality improvement.